1. Field of the Invention
The present invention generally relates to engine starter gearing for an engine. More specifically, this invention relates to engine starter gearing of a positive shift type including a dentil clutch to provide driving and overrunning features and provisions for effecting the automatic separation of the clutch teeth after the engine becomes self-running, and further including means for improving the ability of the engine starter gearing to dissipate the peak torque imposed upon the engine starter gearing during its operation.
2. Description of the Prior Art
The present invention is an improvement over the starter gearing system described in U.S. Pat. No. 3,263,509 entitled "Engine Starter Gearing", issued Aug. 2, 1966, to Digby. The starter gearing system taught by Digby is used to engage and drive an engine ring gear for purposes of starting an engine.
The Digby patent disclosed an engine starter gearing using centrifugal weights and a conical thrust washer for separating dentil clutch teeth after engine start-up to prevent long periods of clutch overrunning and accompanying deleterious wear on the clutch teeth. A driving clutch member and a driven clutch member have complementary mutually engageable inclined dentil teeth for transmitting torque therebetween in one direction of relative rotation. An annular recess is formed in the driving clutch member and a circular recess is provided in the face of the driven clutch member facing the driving clutch member. An annular thrust washer is fitted in the annular recess and abuts the driving clutch member. A conical surface is provided on the annular thrust washer facing the driven clutch member. The centrifugal weights are also disposed in the circular recess and each are provided with an inclined surface which coacts with the conical surface in the annular thrust washer such that when an overrunning condition occurs, the centrifugal weights move radially outwardly and the inclined surface engages the conical surface of the annular thrust washer so as to bias the driving clutch member away from the driven clutch member.
The above described components are all enclosed within a housing through which extends a power shaft. The housing has an open end and a closed end which circumscribes the power shaft. Both the driving and the driven clutch members are concentrically located within the housing on a sleeve which extends through the housing, encasing and slidably splined to that portion of the power shaft which resides within the housing. The driving clutch member is centrally positioned within the housing and has helical splines which engage mating helical splines on the circumference of the sleeve. The driven clutch member is positioned at the open end of the housing and, while being prevented from axial displacement relative to the housing by a snap ring, is rotatably restricted only by the engagement of its dentil clutch teeth with those of the driving clutch member. A pinion which is engageable with the engine ring gear is rigidly attached to the driven clutch member opposite the driving clutch member. A compression spring is used to bias the driving clutch member against the driven clutch member to maintain engagement of the dentil clutch teeth.
For purposes of absorbing torsional shock loads imposed by the engine ring gear on the engine starter gearing, Digby taught the use of a resilient cushion positioned between the closed end of the housing and a radial shoulder of the sleeve located between the closed end and the driving clutch member. A disk is positioned between the resilient member and the shoulder such that it compresses the resilient member against the closed end of the housing. The disk also forms the base from which the compression spring is biased against the driving clutch member. The resulting structure is substantially rigid due primarily to the precompressed resilient member.
Most importantly, the resilient member absorbs the torsional shock transmitted from the engine ring gear during start-up of the engine. Common sources of such torsional shock include the torsional resistance of the engine ring gear when the engine starter gearing first engages the stationary engine ring gear and when the engine ring gear momentarily stops as a result of engine misfire. As a consequence of the driving clutch member being engaged with the sleeve via the helical splines, the torsional shock load is transformed into an axial shock load which results as the driving clutch member, and consequently the pinion and driven clutch member via the meshed dentil clutch teeth, advance toward the engine ring gear. As a result of the axial displacement of the driven clutch member being restricted relative to the housing, the housing is also urged toward the engine ring gear. This displacement of the housing further compresses the resilient member against the disk. In so doing, the original torsional shock load is almost completely transformed into an axial shock load which is primarily absorbed by the resilient member.
The absorption of the axial shock load by the resilient member is desirable in that it reduces the measurable torsional shock sustained by the power shaft, the sleeve and the driving and driven clutch members. Without such protection, the resulting peak torque loads could otherwise exceed the strength of the power shaft and the power shaft splines which engage the sleeve. The result would be in a worst case scenario the cataclysmic failure of the engine starter gearing, and at a very minimum the inability of the engine starter gearing to slidably traverse the power shaft to engage the engine ring gear as a result of plastic deformation of the power shaft splines. While the engine starter gearing of Digby has been satisfactory in operation, it would be desirable to further reduce the effect of the torsional shock load upon the mechanical drive components of the engine starter gearing.
Such an attempt is disclosed in U.S. Pat. No. 3,915,020 to Johnson which teaches another starter gearing structure using a large, annular-shaped resilient member which circumscribes a first portion of the starter housing while being enclosed within a concentric second portion of the starter housing. Such a resilient member, being larger than the resilient member taught by Digby, theoretically has the potential for a greater ability to absorb axial shock loads transmitted from the ring gear. However, assembly of the starter gearing taught by Johnson is complicated by the requirement for the two-piece housing construction. Another disadvantage is that the physical size of the housing is larger than many applications can accommodate.
Therefore, what is needed is an improved engine starter gearing which employs a mechanical feature capable of improving the ability of the starter gearing to absorb the axial shock load transmitted via the clutch members and housing. Furthermore, what is needed is such a mechanical feature which is readily assemblable within the existing housing and which does not negatively affect the performance or function of the existing components.